The present disclosure generally relates to a system for detecting a motion vector of an entire screen caused by vibration or shake of a monitoring camera based on an image signal, and a system for compensating for unwanted sway in an entire screen caused by vibration or shake of a monitoring camera.
FIG. 10 is a block diagram illustrating an image stabilization system employing a conventional motion vector detector. Reference character 1001 denotes an input terminal of an image signal. Reference character 1002 denotes a region vector detection unit for each detection region. Reference character 1003 denotes a vector reliability determination unit. Reference character 1006 denotes an image vector determination unit. Reference character 1007 is a stabilization unit.
In the image stabilization system with the above configuration, image signals of at least two successive frames are first input to the input terminal 1001. In the region vector detection unit 1002 for each detection region, four vector detection regions 1101, 1102, 1103, and 1104 are defined beforehand in an image as shown in FIG. 11, and for each of the detection regions of two frames, the differences Σ|ΔL(i, j) between a signal before a shift by predetermined values (i, j) (where imin<i<imax, j<min<j<jmax) and a signal after the shift are obtained as correlation values. Then, the minimum values (i′, j′) of the correlation values are detected, and used as a vector of a detection region. In addition, the minimum, average, and maximum values of the correlation value are obtained, and output to the reliability determination unit 1003 and the image vector determination unit 1006, together with the vector. As shown in the flowchart of FIG. 12, the reliability determination unit 1003 and the image vector determination unit 1006 determine the reliability of a vector based on the minimum, average, and maximum values of the correlation value of each region, and if the vectors of at least two regions determined to have reliability match each other, the average value of these vectors is determined as a motion vector of an entire screen, and is output. On the other hand, if these vectors do not match each other, a zero vectors is output. The stabilization unit 1007 includes a storage means for storing image signals of one or more frames. Sway is compensated by controlling the read-out location at which an image signal is read out from the storage means. Specifically, as shown in FIG. 13, when an image showing a large degree of sway is input, for input image signals of the (n−1)th frame 1301 and the n-th frame 1302, based on a motion vector 1303 input from the image vector determination unit 1006, a screen of an image signal which is to be read out in a direction 1304 in which a motion in the screen is compensated for is moved in parallel from a position 1305 to a position 1306. The screen is subjected to operations such as enlargement by, for example, interpolation, and image signals 1307 and 1308 each corresponding to the size of one screen and showing a small degree of sway are output (see, for example, Patent Japanese Patent Publication No. S61-269475).
The above-described technique determines the reliability of a vector based on the minimum, average, and maximum values of a correlation value. There are also other techniques for determining the reliability of a vector based on the state of a correlation value, such as a technique using the difference between the minimum value of the correlation value and a vicinity value of the minimum value, i.e., a gradient around the minimum value, and a technique using only the minimum value. There is also a technique in which the reliability of a vector is determined in term of weight, and a weighted mean of vectors is output as a motion vector in an entire screen.
In addition, in a conventional motion vector detector, when a moving object enters a screen, a zero vector is output or a vector to which motion of the moving object is added is detected in many cases. In view of this, a conventional stabilization system is configured to prevent an image from being corrected in a direction different from a direction intended by a camera operator because of interruption of correction or detection of a vector to which motion of the moving object is added. Specifically, as shown in a flowchart of FIG. 14, a plurality of detection regions are provided in a screen so that a motion vector is obtained for each of the detection regions and the reliability of the motion vector is determined. Then, based on the degree of matching with a motion vector of the entire screen, the determination result is changed from the absence of reliability to the presence of reliability so that the reliability of the motion vector is determined. In addition, as shown in FIG. 15, in another conventional stabilization system, a plurality of detection regions 1501, 1502, 1503, and 1504 are arranged in a staggered pattern not to be aligned in any of a vertical direction or a transverse direction, motion vectors are obtained for these detection regions, and the reliabilities of the motion vectors are determined. In this manner, even when a moving object passes through the screen, image stabilization in terms of time is not interrupted, and a malfunction in which an image is corrected in a direction not intended by a camera operator (see, for example, Japanese Patent Publication No. H02-157980).